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*

CCNet DIGEST, 14 June 1999
--------------------------

POEM OF THE DAY

THE ICE CORE (after reading
Nature, June 3 1999)

Sweating in their polar gear at minus
fifty
the drillers extract and rack the cores
of ice,
four hundred thousand years of climate
locked in frozen water rich with
dissolved gas,
dust, ash, samples of air, debris from
space.
Half a world from Vostok the careful
probes
reveal ice ages, climate changes, past
snapshots
of a world we never saw, but which may
come again.
At intervals of a hundred thousand years
the ice
returned, and after its reluctant long
retreat
the greenhouse gases surge to earlier
levels -
is this the clue we seek, cause and
effect,
or was it simply that a colder Earth
resulted
in less production of methane and the
rest?
Why is the present quantity of
greenhouse gas
much higher than it ever was? What
have we done?
Do slow changes in Earth's orbit bring
the ice
or could it be that other source we fear
now,
a fimbulwinter caused by dust and smoke
and burning forests after some
destructive impact?

Hardly a week has passed since Andrea Milani presented his
calculations
for a potential earth-impact of asteroid 1998 OX4 in the year
2046.
Most of us who listened to his report pretty seemed convinced
that this
latest PHA with a non-zero collision probability would not make
the news headlines. The reason seemed obvious enough: The chances
of
OX4 impacting were one in ten million - simply too small to
attract any
media attention. Yet in the debate that followed Andrea's
exposition,
most participants failed to realise that the *real* story of OX4
was
not so much the rather minuscule value given for its impact
chances.
Instead the fact that this potentially hazardous asteroid was
lost
after just 9 days of observations featured prominently in last
week's
discussions and some news reports in the UK.

I believe that these deliberations and reports about 1998 OX4 and
how
it was lost will bring about a whole list of new questions,
questions
which differ considerably from previous debates about XF11 or
AN10
(i.e. PHAs with diameters larger than 1km).

Robert McMillan, the Principal Investigator of the Spacewatch
Project which
discovered OX4 back in 1998 (the discovery was made by Jim Scotti
of
XF11 fame), gives a thought-provoking explanation as to "why
1998 OX4
was lost" (see below). Bob's comment raises a number of
interesting
questions about whether or not 'smaller' NEOs, such as OX4, are
worthwhile searching or following up - given that there may be as
many
as 10,000 to 50,000 objects in this size-category (i.e.
300-600m).

I noticed, however, that in his statement Bob McMillan does not
refer
to the fact that OX4 is of particular interest mainly due to its
non-zero impact probability rather than its size or its nature as
one
of many PHAs. In fact, it is this small impact risk which is the
main
reason why *losing* it - and the prospect of losing similar
objects in
the future - poses a real problem for the NEO search community.
Brian
Marsden, whose most enlightening analysis of the impact threat
posed by
asteroid 1997 XF11 has brought about the current understanding
and
debate of resonant-return impact risks, not surprisingly
underlines
that "we have reached the point were [follow-up programs
are] the
single most important observational need" (see his comment
below).

Another question raised is that of the 'background' hazard for
NEOs in
this size-category. According to current impact rate estimates,
objects
with a diameter of ~300m hit the earth on average every 4000-5000
years. Although the damage caused by 300m asteroids is 'only'
regional
(according to David Morrision they can wipe out a small country -
and
produce craters up to D = 6km), asteroids of this size are much
more
dangerous to us because i) there are many more of them and ii)
because
they will consequently impact more frequently. Obviously, the
'background' probability used for calculating the impact risk of
1998
OX4 was significantly lower than the values used for XF11 and
AN10. It
is no surprise that losing such an object is perceived as an
"embarrassment" (see Nigek Hawkes' article in today's
TIMES).

In short, it would appear that we are facing what I would call
the
"Spaceguard Paradox": while the main Spaceguard goals
(and its main
search programmes) are focusing on the detection of NEAs with
diameters
> 1km, the most likely objects to actually threaten and hit us
in the
*forseeable* future are those which diameter smaller than 1km. In
fact,
asteroids in the OX4 class are the most dangerous of all given
both
their devastating impact effects *and* their impact frequency. If
this
interpretation of our current dilemma is fairly correct, could it
be
that our current priorities are wrong? Of course, one could argue
that
by searching the sky for >1km NEAs, we will inevitably come
across many
objects smaller than 1km. Nevertheless, the whole issue of search
and
follow-up strategies, access to larger and more telescopes, etc.
could
be affected by these questions raised by the OX4 debate.

Tomorrow, an important Spaceguard debate will take place in the
House
of Lords, Britain's Upper House in the heart of London. One can
only
hope the British Government is wise and far-sighted enough to
realise
that there is a growing need for a National Spaceguard Centre.
After
all, neither the exploding list of newly discovered PHAs, nor the
overall impact risk will go away. Most likely, the public
interest in
and the demands for adequate search strategies will become
louder.
Surely, the logical and rational consequence of recent PHA
discoveries
(and losses) is to significantly increase and widen our NEO
search and
research efforts. A Spaceguard Centre in Britain would be an
appropriate first step to respond to the cosmic challenge we are
facing.

Benny J Peiser
14 June 1999

======================================
(2) WHY WAS THE EARTH-APPROACHING ASTEROID 1998 OX4 LOST, AND WAS
IT A
BIG DEAL TO LOSE IT?

Your readers may be interested in why 1998 OX4 was lost. It was
not
through lack of trying. The object was discovered on 1998
July 26 (UT)
by Jim Scotti with the 0.9-m Spacewatch telescope on Kitt
Peak. Those
who are familiar with the climate of southern Arizona know that
it can
be difficult to observe in the summer owing to frequent (although
localized) thunderstorms.

The object was V mag 20.7 at discovery, and although our
discoveries
appear promptly on the Minor Planet Center's NEO Confirmation
Page, we
are very conscious that many recovery observers cannot reach that
faint, especially on a fast- moving object with an uncertain
ephemeris.
So a fast-mover such as this does get some priority at Spacewatch
at
the expense of scanning for further discoveries.

Jim reobserved the object on two epochs the following night, but
was
clouded out on the 28th. By the 31st the Spacewatch
observer was now
Dr. Jeff Larsen, who was ready to open and be on the object
within 5
minutes, but was still clouded out on Kitt Peak. However,
thanks to
the localized weather pattern, it was clear at nearby Mt. Hopkins
where
Carl Hergenrother recovered the object again with the 1.2-m
telescope
there.

Kitt Peak remained completely clouded out through the rest of the
dark
run in early August, except for Jeff's attempt at recovery on
Aug. 3 UT

which was aborted by clouds after he opened. Dave Balam
recovered the
object on 1998 Aug 3 and 4 UT with the 1.8-m telescope of the
Dominion
Astrophysical Observatory in Victoria, BC, Canada.

Spacewatch's next opportunity to observe was Aug. 20, but by then
the
object (at 20.5 magnitude) was immersed in the Milky Way. By the
September run, 1998 OX4 was too far west to observe. So Balam's
observations were the last to be reported to the MPC.

With the help of Bruce Koehn of LONEOS's fine website at Lowell
Observatory (http://asteroid.lowell.edu/cgi-bin/koehn/obsstrat)
we see
that the 1 sigma uncertainty in 1998 OX4's position in 1999 March
when its magnitude peaked at about V=21.6 was a few thousand
arcseconds. That means too much sky area to search for such a
faint
object. At the next opposition passage the uncertainty will be in
the
10's to 100 thousand arcseconds (1 sigma again), though it peaks
at
about V=20. It'll again be in the Milky Way then. It's fainter
than
V=25 at the present time.

Is it worthwhile to discover objects during a season of
persistently
bad weather? I think so, because even if the objects cannot
be
recovered, the discoveries contribute to the accumulation of
statistics
on the distributions of asteroids with absolute magnitude and
orbital
parameters, and to an extrapolation to their total number.
Jedicke
and Metcalfe (1998 Icarus 131, 245-260) showed how much can be
done
with very short arcs on a number of main belt asteroids.
Spacewatch
will be doing similar analyses on its discoveries of
Earth-approachers.

Is it worthwhile to discover objects that faint when the recovery
network is so leaky at those magnitudes? This is a
"chicken-or-the-egg"
problem: if no asteroids were discovered fainter than 20th
mag then
there would be no motivation for recoverers to reach fainter. In
any
case, reaching fainter tends to explore the distribution of
smaller
sizes, which leads to the next question:

Are objects as "small" as 1998 OX4 important enough to
follow,
considering that there could be 10,000 objects of this size and
that Spacewatch is not finding such small objects fast enough to
significantly mitigate the hazard of their impacts? I think they
are worth discovering and following for scientific reasons. The
distribution of sizes of small asteroids holds clues to the
cohesive
strength of asteroid material (Durda, Greenberg, and Jedicke 1998
Icarus 135, 431-440). Small asteroids may also rotate
rapidly, and such
rotation also provides information on the binding strength of
asteroid
material (Pravec 1999 ACM abstract).

How can losses such as 1998 OX4 be prevented? For the
moment I'll put
aside the question of whether it is worth following up every
faint Earth
approacher; there will always be some objects at the limit of
detection
that will get lost, so one can question whether precious
observing
resources should be overly concentrated on that one aspect of the
campaign. Such objects might be more easily rediscovered at
another
epoch when they might be brighter.

Additionally, improving the followup network is too large an
issue for
this email message, so I will restrict my remarks to the
specifics of
the case of 1998 OX4. For much of the time that 1998 OX4
was at southerly
declinations it was also in the Milky Way. This is
inevitable for
low-inclination asteroids. Although Grant Stokes' LINEAR
asteroid
survey group at MIT apparently has advanced software to discover
and
recover asteroids in those crowded star fields, their all-sky
survey
mode does not currently reach faint enough to have recovered 1998
OX4,
nor do I know whether it would be technically feasible or
politically
appropriate for their methodology or software to be distributed
to the
world-wide recovery network.

Certainly an expanded image scale helps in crowded fields, such
as one
obtains with a "large" telescope. The last
observation of 1998 OX4 was
by Dave Balam, not by virtue of a southerly observatory location
(his
latitude is +48.5 degrees!) but thanks to the 1.8-m telescope he
uses.
A "large" (2-m class) telescope in the southern
hemisphere with an
image scale of less than one arcsecond per pixel and good
software for
crowded fields would be effective for recoveries of faint,
fast-movers
in the Milky Way, but again, that would have to be traded against
other
(probably more efficient) ways to accelerate discovery and
follow-up of
Earth-approachers in general.

It was good to see Bob McMillan's
attempt to place the 1998 OX4
situation in perspective. As I have stressed many times, most
recently
at the Turin workshop, extended follow-up at the first apparition
and
recovery at subsequent oppositions require access to
large-aperture
telescopes, in both the northern and southern hemispheres, and
well-enough separated in longitude that vagaries of the weather,
such
as the summer "monsoons" in the southwestern U.S., can
be overcome.
Indeed, many of us involved with the Turin subgroup 1, which was
concerned with search and follow-up programs, felt that we have
reached
the point where this is the single most important observational
need--yet it has traditionally been the area in which the least
amount
of progress is made. Of course, the problem is most severe for
the
intrinsically fainter objects, but it is also applicable for
brighter
objects (e.g., those believed to be relevant to the goal of
discovering
most of the km-sized objects) at distant oppositions.

At absolute magnitude H = 21.3, 1998 OX4
is presumably much
smaller than 1997 XF11 and 1999 AN10, the first two cases where
possible earth impacts were identified during the next half
century.
Indeed, much of the interest in these objects has been because
they are
in the range where an impact would have global
consequences. If we
direct our attention to intrinsically fainter objects, more and
more
cases of noticeable impact probabilities will surely be found. At
one
level, I am delighted that my drawing attention, originally in
the
CCNet on June 8 last year, to the post-2028 danger of 1997 XF11
has
given rise to a "cottage industry" of making such
calculations. At
another, I understand Bob's rather guarded questioning of whether
it is
worth following up objects as small as 1998 OX4. Since it
had H = 22.0
or brighter, 1998 OX4 could qualify as a PHA, and it was
indicated as
one in the discovery announcement on MPEC 1998-O27 on July 31.
The H =
22.0 limit, corresponding roughly to a diameter of 200 meters,
was
selected to include objects that might yield tsunami damage on a
rather
global scale. Even so, the limit is arbitrary in the sense that
there
could always be a fainter, sub-Tunguska-sized object, not
classified as
a PHA, that on discovery yields a 99-percent chance of earth
impact
just a few days later.

When it comes to the impact-probability
calculations, where, if
anywhere, should one draw the line? That is indeed the
question. But
since there are now so many PHAs, with 75 of the 179 having been
discovered during the past 18 months, and with some people
perhaps
wishing to extend the limit to fainter than H = 22.0, it is
really
mainly a question of when the specific possibilities of impact
are
identified. For 1997 XF11 the 2040 possibility was identified
from the
first 88-day arc of observations. For 1999 AN10 the 2039
possibility
was from a 38-day arc, the 2044 and 2046 possibilities from a
123-day arc. The 1998 OX4 case involved only a 9-day arc. Of
course, it
is always in principle possible to recognize the possibility of
an
impact when the observations cover a smaller arc, but the orbital
uncertainty then means that the actual probability of an impact
decades
later will also be smaller. However, if it had in fact been
possible to
extend the Spacewatch observations of 1998 OX4 until
mid-September,
thus yielding an arc of some 50 days, chances are that the
possibility
of a 2046 impact, estimated as 1 in 10 million from the initial
9-day
arc, would completely vanish--just as the 2040 impact possibility
for
1997 XF11 disappears with the consideration of the 1990 data.

But that's not what happened with 1999
AN10, when -surprisingly-
Frank Zoltowski's recovery observations confirmed a close
approach in
2027 and substantially enhanced the likelihood of impact during
the
following two decades. Furthermore, if the 2046 impact
possibility for
1998 OX4 had already been identified during the week or so
following
the publication of the observations from Day 9 on Aug. 7, a
special
campaign for further August-September observations could have
been
launched, using appropriately large telescopes.

It's a tricky problem. While there is
clearly merit to following
up PHAs, as well as other NEOs, both astrometrically and
physically,
with too many computations of non-zero impact probabilities from
short-arc orbits, there is the danger of going on too many
wild-goose
chases, with unnecessarily large efforts (including "peer
review"
recomputations of the impact probabilities by others) expended on
objects that turn out, not only not to be any threat to the earth
during the foreseeable future, but not even to be particularly
interesting. Meanwhile, more significant objects could be
ignored.
Against this one weighs the reassurance that the additional
observations in most instances will render an object harmless. As
with
the other dangers we face daily, some reasonable balance is in
order.
Since 1998 OX4 is one of very few (i.e., two) objects for which
non-zero impact probabilities are still extant for upcoming
decades, I
suggest we simply tag it as of some interest to the astronomical
community, but about which nothing can be done unless it is
accidentally rediscovered.

THE good news (sic) is that astronomers have identified an
asteroid
that could be on a collision course with Earth. The bad news is
that
they have lost it.

The object, called 1998 OX4, was found last year by a team at the
University of Arizona, who tracked it for two weeks.

The information the scientists gathered gave an approximate orbit
for
the object, which is believed to be several hundred yards in
diameter
and capable of continent-wide destruction if it were to collide
with
Earth.

A team at the University of Pisa, led by Dr Andrea Milani, used
the
orbital information to calculate the chances of a collision. He
told a
conference in Turin last week that there was a one-in-ten-million
chance that it would hit the Earth in 2046 - slightly better odds
than
the 14 million to one on winning the National Lottery.

The asteroid is only the second such object found with a non-zero
chance of a collision. And while a one in ten million chance is
not
enough to lose sleep over, it is embarrassing that 1998 OX4 has
now
been lost. The problem is that the orbit was not known with
sufficient
accuracy to track it. If it could be found, the chances are that
more
precise observations would dispel the fear of a collision.

Duncan Steel, a British astronomer who specialises in watching
out for
potential asteroid collisions, says that the dilemma posed by
1998 OX4
will become increasingly common. "The searches being done in
the US are
finding more and more objects that could potentially collide with
Earth - so many that there aren't enough people to keep track of
them,"
he says.

"The rate at which these objects are found has increased by
three or
four times in the past few years. False alarms are getting more
frequent. Where we fall down is in following up on the original
finds."

He is not too concerned about 1998 OX4. "But sooner or later
we are
going to find one with a one-in-a-thousand chance of colliding,
and
that could set off a panic," he says.

If it happened tomorrow, the Government would have to rely on
experts
overseas to give a proper assessment of the danger. "We need
much
better machinery in Britain to take responsibility for giving
advice,"
he says.

Next week the Liberal peer Lord Tanlaw will ask a question in the
House
of Lords, seeking to discover whether the Government has any
plans to
establish a national "Spaceguard" programme of
observation for
near-Earth asteroids.

Dr Mark Baily of Armagh Observatory says that he believes such a
programme is necessary and inevitable. "There are going to
be more and
more of these objects found. If we don't have a national agency,
advice
on the real level of risk will have to come from astronomers who
do
this sort of work in their spare time, or from those with no
allegiance
to the United Kingdom."

The lesson of 1998 OX4 is that, having found near-Earth objects,
they
should not be lost again, he says. "I won't lose any sleep
over it," he
says. "The risk it will hit us is much less than the risk of
a totally
unknown object doing so. All these calculations show is that we
should
go on tracking it - if we knew where it was."

Dr Steel does not rate highly the chances of finding 1998 OX4
again.
"It's like finding a needle in a haystack, and then throwing
it back in
again," he says.

It contains published data on all known NEAs and the
corresponding
bibliographic references.

The idea is to update these pages continuously, i.e. add data for
new
discoveries as soon as available, the entries for newly published
data
will be made on a monthly or so basis. In order to keep the
entries up
to date, cooperation with the researcher working in the field is
essential and therefore it would be greatly appreciated if new
data
are brought to our attention as soon as possible.

Looking forward to comments and/or suggestions for further
improvement
or extension of this service and its possible integration into
other
databases.

* Nature of the Impact Hazard: For any given size (energy) of
potential
impactor, there is a "background probability" of impact
from unknown
objects. As more NEOs are discovered, this background probability
decreases. However, occasionally a newly discovered NEO is found
to be
on an orbit that repeatedly brings it close to the Earth, and
that has
a non-zero chance of impact at one or more discrete times in the
future. As the orbit is refined, these discrete moments of risk
will
generally disappear. There are no more than a handful of truly
threatening NEOs (D >1 km) in any century, and perhaps
none. The
progress of Spaceguard can then be thought of as a replacement of
a
general background risk with discretely identified risks from a
very
small number of NEOs, which will of course be carefully tracked
to
determine their future orbits with high precision.

* Appreciation of the Risk: Although the public is broadly
aware of
the impact hazard, and there has recently been evidence of
increased
interest in the U.S. Congress and the UK Parliament, it appears
that
the reality of the impact hazard has still not been accepted by
many
decision-makers, including most professionals in the risk
assessment
profession. Geof Sommer of RAND provided the workshop a
provocative
discussion of how we might formulate some of our issues in terms
that
can communicate better with policy makers and perhaps enhance the
credibility of NEO impacts as a risk issue.

* Search and Discovery: The rate of discovery of NEAs has greatly
accelerated, with the bulk of the recent discoveries coming from
the
MIT LINEAR program using a single 1-m telescope. Grant
Stokes reported
that a second identical LINEAR telescope is about to begin
regular
operations, and other systems are also working, as described in
previous NEO News notes. However, to meet the Spaceguard
objective of
discovering 90% of NEAs >1 km in diameter by 2009, it will be
necessary
to extend the search down to approximately visual magnitude 20.5,
which
has not been demonstrated for LINEAR or other systems that use
1-m
telescopes. Thus it is not yet clear whether an expanded network
of 1-m
telescopes can do the full job.

* Follow-up Observations: NEA discoveries must be rapidly
followed up
to determine orbits. Many groups, including amateur astronomers,
now
contribute to follow-up observing programs. This work is quite
effective, but most of the present observers do not have large
enough
telescopes to observe discoveries that reach to magnitude
20.5. Thus
as the discovery rate of faint NEAs increases, there may be a
crises in
follow-up. We also lack follow-up capability in the Southern
Hemisphere, which could lead to the loss of many NEAs that are
moving
south at the time of discovery.

* Availability of data: As the number of NEA observers increases,
and
as more people have the capability to calculate orbits and impact
probabilities, it is essential to move toward more rapid
dissemination of
data on NEA positions. Probably a system can be developed soon to
provide automatic, essentially instantaneous posting of
observational
data on the Internet.

* Cooperation and Coordination: A successful Spaceguard program
requires detailed coordination of observations to avoid
redundancy and
make full use of the available resources. Some observers
are already
posting their observing plans on the Internet. Better
coordination
will be required, however, as the rate of discovery continues to
increase.

* Physical Characterization: There is a continuing need for
physical
characterization of NEOs, primarily using ground-based telescopes
and
radar. In addition, a number of spacecraft missions to
comets and
asteroids are planned or underway, which should greatly increase
our
knowledge of the nature of these objects.

* Impact Hazard Scale: A new Torino Impact Hazard Scale,
developed by
Rick Binzel, was endorsed by attendees at the workshop.
This scale,
ranging from 0 (risk well below background level) to 10 (certain
catastrophic impact), will be described in detail in a future
message.

* Verification of Threatening NEOs: The workshop attendees
recommended
that the International Astronomical Union take responsibility for
establishing a system for voluntary rapid peer review of
predictions or
announcements of any NEO with significant impact risk (level 1 or
higher on the Torino risk scale). This review will also be
described
on NEO News when the IAU works out the details.

Dr. Christophe Laux of Stanford University has proposed to create
a
special session on meteors at the next 38th Aerospace Sciences
Meeting&Exhibit of the AIAA (American Institute of
Aeronautics and
Astronautics). The meeting will be held at Reno, NV, from
January
10-13, 2000.

The special session will be entitled "Aerothermochemistry
effects in
meteoric plasmas" and will be chaired by meteor astronomer
Dr. Peter
Jenniskens of The SETI Institute at NASA/Ames Research Center and
co-hosted by plasma physicists Dr. Olga Popova of the Moscow
Institute
for Dynamics of Geospheres RAS and Dr. Iain Boyd of the
Department of
Aerospace Engineering of the University of Michigan. It will be a
half
day session and will feature a maximum of eight half-hour oral
presentations.

The AIAA meeting is usually very well attended (1600 participants
last
year). This would be a tremendous opportunity for the
plasmadynamics,
thermophysics, and fluid dynamics communities to learn more about
the
physics, aerochemistry, and optical diagnostics of meteorids.
Many
participants at the AIAA meeting work in the area of hypersonic
flows,
plasma chemistry and plasma diagnostics. I trust this would be a
great
venue to present the exciting findings that were described at the
last
Leonid conference as well as some first results from the 1999
Leonid
campaign.

The deadline for abstracts for this conference for this
particular
session is June 22. Because of the short notice, all abstract
requirements are waived. At this time, we will collect proposed
titles
for presentations only. No

abstract needs to be submitted. Note that all presenters must
submit a
written paper by the first day of the conference.

We will need the following information for each paper (by June
22,
1999):

- Paper title
- List of all authors with their affiliations (e.g.: NASA-Ames,
Moffett
Field, CA, USA)
- Name, address, phone, fax and email of the one author who will
receive
all correspondance
- Indicate any special needs for video equipment (VCR, 35 mm
projector)
[the default is an overhead projector in each room].

THE FOLLOWING CORRECTIONS APPLIES TO THE PRESS RELEASE ORIGINALLY
SENT
OUT ON JUNE 10, 999

1. THE QUOTE ORIGINALLY ATTRIBUTED TO PAUL CHODAS ON THE JPL WEB
SHOULD
HAVE BEEN ATTRIBUTED TO ANDREA MILANI

2. THE EXPECTED MISS DISTANCE OF 1999 AN10 WAS INCORRECTLY STATED
OF
39,000 KM; THE CORRECT MOST LIKELY MISS DISTANCE IS 200,000
KILOMETERS.

WE APOLOGIZE FOR ANY CONFUSION THESE ERRORS MAY HAVE CAUSED.

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